1. Field of the Invention
The present invention relates to a method for manufacturing a polymerized toner, and more specifically to a method for manufacturing a polymerized toner that has a high quality and excels in the stability of the quality by adopting a washing manner that enables uniform washing efficiency without insufficient washing portion using a relatively simple washing operation, and excels in the reproducibility of the washing efficiency, in a step for washing colored polymer particles after polymerization.
2. Description of the Related Art
In image forming machines such as electrographic or electrostatic copying machines, laser-beam printers and facsimiles, a developer is used for visualizing electrostatic latent images formed on a photoconductive member. The developer contains colored particles (toner) containing a colorant, a charge control agent, a release agent and the like dispersed in a binder resin as the major component thereof.
Toners are roughly divided into pulverized toners obtained by a pulverizing method, and polymerized toners obtained by a polymerization method. In the pulverizing method, a thermoplastic resin are melted and kneaded together with additive components such as a colorant, a charge control agent, and a release agent, and the resulting mixture is pulverized and classified to obtain a pulverized toner as colored resin particles. The pulverized toner has an indeterminate form and a broad particle diameter distribution. Moreover, since fine particles are easily formed by pulverization in the pulverized toner, it is difficult to manufacture a small particle diameter toner having a desired average particle diameter at a high yield.
In the polymerization method, a polymerizable monomer composition containing a polymerizable monomer, a colorant, and other additive components is polymerized in an aqueous dispersion medium to form a polymerized toner as colored polymer particles. In the polymerization method, the formed colored polymer particles are washed, filtered, and dried after polymerization to obtain a polymerized toner.
According to the polymerization method, a spherical polymerized toner with a sharp particle diameter distribution can be manufactured. Also according to the polymerization method, core-shell structure colored polymer particles can be formed by polymerizing a polymerizable monomer for the shell after the polymerization of the polymerizable monomer composition, in the presence of the formed colored polymer particles. If the glass transition temperature of the polymer component constituting the core is lowered, and the glass transition temperature of the polymer component constituting the shell is elevated, a polymerized toner that excels in shelf stability and low-temperature fixing properties can be manufactured. Furthermore, according to the polymerization method, a polymerized toner having a volume average particle diameter of 10 μm or less, or as further smaller particle diameter as 3 to 8 μm, can be easily manufactured. The polymerized toner, can therefore form a high-definition and high-quality image and is suitable also for increasing a printing speed and forming a full-color image.
However, since the polymerized toner is formed as colored polymer particles by polymerizing a polymerizable monomer composition in an aqueous dispersion medium, the surfaces thereof are easily affected by various components dispersed or dissolved in the aqueous dispersion medium. For example, an aqueous medium containing various dispersion stabilizers is generally used as the aqueous dispersion medium, and the dispersion stabilizers adhere on the surfaces of the formed colored polymer particles. Also, a charge control agent for positive charge or negative charge is generally contained in the polymerizable monomer composition to improve the electrostatic properties of the polymerized toner; if the charge control agent has a high polarity, a part thereof is dissolved in the aqueous dispersion medium, and adheres on the surfaces of the formed colored polymer particles.
If various components adhered on the surfaces of the polymerized toner particles are not removed sufficiently and uniformly in the washing step after polymerization, the charge amount distribution of the polymerized toner becomes broad, causing image densities to lower and causing fog easily, especially under high-temperature and high-humidity conditions. For that reason, in the method for manufacturing the polymerized toner, various manners for washing colored polymer particles (polymerized toner particles) formed in the polymerization step have been proposed.
For example, in Japanese Patent Laid-Open No. 2000-10341, there is proposed a method for manufacturing the polymerized toner particles wherein the filtration of colored polymer particles from a liquid medium after polymerization is performed using a filter press. In this manner, the colored polymer particles after filtration are washed with water.
In Japanese Patent Laid-Open No. 2000-310887, there is proposed a method wherein colored polymer particles after suspension polymerization are washed with an acid, dehydrated, and then the colored polymer particles are washed with warm water. In this manner, the colored polymer particles are washed with warm water at a temperature, not lower than 35° C. and not higher than Tg (the glass transition temperature of the colored polymer particles) minus 10 degrees centigrade (Tg-10° C.), dehydrated, and then dried.
Furthermore, in Japanese Patent Laid-Open No. 2002-148860, there is proposed a method for manufacturing a polymerized toner, wherein colored polymer particles after polymerization are filtered from the medium, a washing liquid is added to the filtered colored polymer particles to form a slurry, the colored polymer particles are filtered again from the slurry, and then the filtered colored polymer particles are dried.
However, in the manner for washing colored polymer particles during fixed on a filter medium, since a washing liquid is passed through a cake layer of a certain thickness comprising colored polymer particles, the washing effect may be insufficient or uneven. In the washing manner by repeating dehydration and re-slurry, the operation is extremely complicated, and the installation area of the apparatus becomes inevitably large.